1. Field of the Invention
This invention relates to a printing ink, thermal transfers prepared with the printing ink and methods of using the thermal transfers to decorate polyolefin articles.
2. Brief Statement of the Prior Art
A vast variety of articles are molded from polyolefin resins, particularly polyethylene and polypropylene resins by manufacturing techniques such as injection molding, blow molding, rotational molding and thermal forming. Decoration, including labeling, of articles molded from these resins is often not permanent, as the resins resist adhesive coatings. A common approach to enhance the adhesion of applied coatings has been to oxidize the surface of articles molded from these resins before the application of coatings. While this oxidation treatment has met with some success, it increases manufacturing time and cost of the molded articles.
Attempts to improve adhesion of labels to untreated surfaces of articles molded from polyolefin resins have included the use of heat activated labels such as disclosed in U.S. Pat. No. 7,622,171 which are applied to the articles as transfers with an adhesive layer of a vinyl acetate resin, a tackifying hydrocarbon resin and a microcrystalline wax. This approach, however, adhesively bonds the label to the article's surface. The label is not integral with the surface, but lies on top of the surface and is subject to wear and delamination.
A related problem with decoration of articles prepared from polyolefin resins by rotational molding has been eliminated with transfers printed with pigments or dyes in oil or wax which have been used as in-mold transfers that are molded into the surface of a polyolefin article as it is formed in a molding cycle and as on-mold transfers which are applied to the surface of preformed polyolefin articles; see U.S. Pat. Nos. 4,252,762; 4,519,972; 5,840,142; 6,613,830; 7,128,970; and 7,641,842. While these transfers have successfully achieved permanent decoration by fusing decorative layers into the surfaces of rotationally molded articles, they are not readily compatible with equipment used for high speed printing and decorating of mass produced polyolefin articles.